Simple is Good

In the 'old' country (NZ) my grandparents used Hotwater Bottles that were flat pottery bottles with a stopper. (As a child I thought they were stones used for Curling!)







I rediscovered the modern Hotwater Bottle a few years ago.
Just like the mouse trap, the design hasn't changed for decades, and it can't be improved on.

The instructions say not to use boiling water, but it works best if you do. Put in as much water as you want, then squeeze the air out before tightening the stopper. One side of the 'bottle' is thicker and cooler than the other. Use that side first and then flip it over after an hour or so. Put one in the bed an hour before you get in. Place one on your feet under your desk.
When you're finished with it, hang it upside down on a hook to dry.
Simple is good.